Continuous phase modulation (CPM) signals are phase-modulated signals having a spectral occupancy that can be tailored to fit limited transmission bandwidth through suitable pre-modulation filtering. Moreover, unlike non-constant envelope signals such as amplitude-modulated signals or filtered phase-modulated signals, the CPM signals are of constant envelope and allow saturated power amplifier operation for maximum power efficiency. These desirable signal properties, fueled by the rising premium being placed on bandwidth and power efficiency, have resulted in CPM signals such as binary Gaussian Minimum Shift Keying (GMSK) being deployed in operational terrestrial and satellite communication systems. The ever-rising premium of bandwidth efficiency further motivates extending binary CPM to higher alphabet signaling format such as quaternary GMSK.
A quaternary CPM signal, despite its relatively poorer power performance than binary CPM, can be an attractive signaling format when used in conjunction with some forward error correction schemes. The use of soft-decision error correction decoding is particularly desirable in such a coded CPM communication system as it can significantly reduce the signal-to-noise ratio (SNR) needed for achieving an overall error rate performance. This SNR improvement, however, is predicated on the ability of the underlying demodulator to generate appropriate soft bit metrics for the soft-decision decoder. Conventional quaternary CPM signal demodulator produces hard decisions on symbols, hence on bits, by applying the maximum likelihood sequence estimation (MLSE) Viterbi algorithm and identify the most probable symbol sequences. Using such a hard-decision symbol demodulator in a coded quaternary CPM communication system brings only sub-optimal performance as only hard-decision error correction decoding is permissible.
Mengali taught reduced-complexity quaternary CPM demodulator based upon the pulse-amplitude modulation (PAM) components of the complex envelope of a quaternary CPM signal as an extension of the PAM-based reduced-complexity receiver originally proposed by Kaleh for demodulating binary CPM signals. The design principle of such a reduced-complexity quaternary CPM demodulator is footed on a bank of matched-filters associated with the PAM components of the underlying CPM signal. The complexity reduction is achieved by using only a subset of the PAM matched-filters in the receiver. The total number of PAM components in a quaternary CPM signal is equal to 3·4(L−1) where L represents the memory of the CPM pre-modulation filter. The memory length L is generally in the order of the reciprocal of the bandwidth-time product BT, that is, L=1/BT. Typically, for moderate bandwidth-time product BT, only a small subset of these PAM components needs to be considered for demodulation purpose. For example, with BT=1/3, only the first three energy-dominate PAM components in the quaternary GMSK signal need to be considered out of a total of forty-eight PAM components, and hence, only three respective matched-filters are used for demodulation. Following Kaleh's work on demodulating binary CPM signals, Mengali taught hard-decision quaternary symbol demodulation using MLSE Viterbi algorithm. However, both Mangali and Kaleh have failed to construct an appropriate soft bit metric using the theoretical log-likelihood ratios and as such are less than optimal in performance. Additionally, Mangali and Kaleh taught hard-decision symbol demodulation requiring extensive memory elements for storing the survivor path states when implementing the MLSE Viterbi algorithm. These and other disadvantages are solved or reduced using the invention.